The present invention relates to a d-c circuit breaker and more particularly to a d-c circuit breaker which produces current zero to break a d-c current by superposing an oscillating current on the d-c current.
In general, it is difficult to break the d-c circuits as compared to breaking the a-c circuits, because of the fact that unlike the a-c currents, the d-c currents do not have a point at which the current becomes zero. A d-c circuit breaker or an interrupt or disposed in the d-c circuit is often opened by a widely known arc quenching method which connects a capacitor in parallel with the interrupt or to produce a current zero.
The capacitor and the interruptor connected in parallel constitute an oscillatory circuit together with the inductance contained in the parallel circuit. The inductance contains stray inductance in the oscillatory circuit as well as the inserted inductance, and the stray inductance is induced by the wirings and by the capacitor itself. The oscillatory circuit represents an L-C series resonance circuit of the capacitor and the inductance, and the oscillating current i.sub.o is generated by appropriately selecting the values of the capacitor and inductance (oscillating current i.sub.o occurs when the interruptor is opened).
The oscillating current i.sub.o is superposed on the d-c current I flowing from the d-c circuit to the interruptor, and a superposed current i (i=I+i.sub.o) consisting of the d-c current I and the oscillating current i.sub.o flows across the electrodes of the interruptor. If these two currents are so selected that i.sub.o .gtoreq.I, the superposed current i produces a current zero. The arc produced across the electrodes of the interruptor is extinguished when the superposed current i becomes zero. In the aforementioned arc quenching method, the capacitor may be electrically charged to a predetermined potential or may not be charged before the capacitor is connected to the interruptor. In the following description, the former is referred to as a pre-charging method and the latter as a non-charging method.
In the pre-charging method, the capacitor is connected to the interruptor just before or just after the interruptor is opened. In either case, the produced oscillating current i.sub.o is approximately represented by the below-mentioned equation (1), and the amplitude of the oscillating current decreases almost exponentially due to the presence of ohmic resistance in the circuit. ##EQU1## where Ec represents an initial charged voltage of the capacitor,
Lo an amount of inductance of the oscillatory circuit, PA1 C capacitance of the capacitor, PA1 .alpha. a constant, and PA1 t a time.
Japanese Publication of Utility Model Application No. 40-18098 (1965) entitled "D-C Vacuum Circuit Breaker" discloses a breaker in which the capacitor is connected just before the interruptor is opened, and G. A. Kukekor et al "Switching-gear for H.V.D.C. Lines" Direct Current, June 1959, pp. 123-126, discloses a breaker in which the capacitor is connected just after the interruptor is opened.
In the pre-charging method, the following requirements are necessary so that the d-c current is successfully interrupted. ##EQU2## where Imax represents a maximum current that can be interrupted (hereinafter referred to as a maximum breakable current), and di/dt represents a time differential value of the current i when the superposed current i becomes zero (hereinafter referred to as a current slope) and is approximately represented by the equation (4) ##EQU3## and .beta. represents a value specific to each interruptor; the breaking results in failure if the current slope exceeds the maximum current slope .beta..
Therefore, if the initial charged voltage of the capacitor Ec of the equation (2) is increased in order to increase the maximum breakable current Imax, the current slope di/dt given by the equation (4) is increased.
Therefore, in the aforementioned conventional devices, an inductance having an amount of inductance greater than several mH is directly connected to the capacitor so that Lo of the equation (4) will have an amount of inductance greater than several mH, thereby to restrain the current slope di/dt. It is therefore difficult to increase the frequency f of the oscillating current i.sub.o given by the equation (5) to a value above 1 kHz. EQU f=1/(2.pi..sqroot.LoC) (5)
Further, in the pre-charging method, it is impossible to bring the current slope di/dt at the time of breaking into zero, irrespective of the magnitude of the d-c current I that is to be broken. The current slope di/dt can be brought into zero when the magnitude of the d-c current to be broken is in agreement with the amplitude of the oscillating current i.sub.o. With the aforementioned pre-charging method, however, if the magnitude of the d-c current I which is to be broken undergoes variation even when the current slope di/dt is selected to be zero at a particular d-c current, the current slope di/dt tends to be increased.
In the non-charging method, on the other hand, the capacitor is connected in parallel with the interruptor via a spark gap or an auxiliary switch when the arc voltage across the electrodes of the interruptor reached a predetermined value after the interruptor has been opened. The oscillating current i.sub.o in this case corresponds to that of the pre-charging method in which the initial charged voltage of the capacitor Ec is substituted by an arc voltage Va at the time of connecting the capacitor. A maximum breakable current Imax in this case is represented by the following equation ##EQU4##
The upper limit of the arc voltage Va will be about 2 KV. In the non-charging method, therefore, in order to increase the maximum breakable current Imax, the amount of inductance Lo of the oscillatory circuit must be reduced as disclosed in H. Hartel "Nebenwege fur HGU-Schalter" ETZ-A Bd. 91 (1970) H.2, pp. 79 to 82. Therefore, Lo had been selected to be smaller than 5 .mu.H. Accordingly, with the non-charging method, the oscillating current of a frequency f of up to about 10 KHz can be treated, but it is difficult to break the circuit at a current slope di/dt.perspectiveto.0 regardless of the change in d-c current I to be broken, because of the same reasons as those of the case of the pre-charging method.
In the aforementioned arc quenching method, the amplitude of the generated oscillating current i.sub.o is decreased with the passage of time. According to N. Yamada et al "H.V.D.C. Circuit Breakers Using Oscillating Current Techniques" Direct Current, August 1966, pp. 87 to 67, however, the oscillating current with gradually increasing amplitude (hereinafter referred to as divergent oscillating current) is generated by the aforementioned non-charging method. The divergent oscillating current has a region which exhibits such a characteristic that the arc voltage is increased with the decrease in arc current of the interruptor (hereinafter referred to as a negative arc resistance characteristic), and is generated when the d-c current I to be broken lies within this negative arc resistance region. The method disclosed in the above literature of N. Yamada et al employs the divergent oscillating current. With this method, however, since the capacitor is connected in parallel with the interruptor after the arc voltage is raised to a predetermined value, it is impossible to break the circuit at a current slope di/dt.perspectiveto.0 irrespective of the change in d-c current I.
Other relevant prior arts are as follows:
(1) U.S. Pat. No. 3,522,472 entitled "Direct Current Breaker". PA0 (2) Japanese Publication of Utility Model Application No. 40-10355 (1965) entitled "D-C Vacuum Circuit Breaker".
This patent discloses a d-c current breaker which has an oscillatory circuit formed of an interruptor, and a series circuit of a capacitor and a coil connected in parallel with the interruptor. The oscillatory circuit is referred to on column 6, lines 40 to 50 of the specification and in FIGS. 8a and 8b.
This publication discloses a d-c vacuum circuit breaker based on the pre-charging method. The pre-charging method is referred to on column 1, lines 1 to 21 and in FIG. 1.